1. Field of the Invention
The present invention relates to gas sensors, and more particularly, to electrochemically functionalized, nanomaterial-based gas sensors.
2. Description of Related Art
Gas detection instruments or gas sensors have a wide range of applications, including industrial health and safety, environmental monitoring, and process control. Some of the fields in which gas sensors are used include chemical refining, petroleum refining, rocket fuel production, fuel cell manufacturing, semiconductor processing, and biomedical applications. In one example of a biomedical application, hydrogen gas sensors may be used to detect hydrogen in exhaled breath. The presence and concentration of exhaled hydrogen may be used to diagnose various diseases, including lactose intolerance, fructose malabsorption, fibromyalgia and neonatal necrotizing enterocolitis.
Typically, gas sensor reactions occur at elevated temperatures. Therefore, the operating temperatures of both the sensor material and the gas to be detected must be controlled for optimal response, which typically means that the sensors must be heated. In addition, the sensors should have a large ratio of surface area to volume to increase the opportunities for surface reactions. Gas sensors made of nanoscale materials exhibit the desirable large ratio of surface area to volume. For example, single-walled carbon nanotubes (SWNTs) are considered ideal building blocks for making gas sensors, as all the carbon atoms in SWNTs are exposed to the surface, providing abundant surface area per unit volume.
Nanoscale materials (also referred to as “nanomaterials”) are defined as having at least one physical dimension in the range of 1-100 nanometers. These materials, such as carbon nanotubes, can be either semiconductive or conductive, depending on their diameter and helicity. Semiconductive and conductive nanomaterials can exhibit sensitivity to gases. For example, carbon SWNTs are particularly advantageous for making gas sensors, because all of their carbon atoms are exposed to the outer surface of the structure, thereby providing more surface area exposed to the gas. With their unique electrical, thermal, and mechanical properties, SWNTs exhibit better sensitivity compared to conventional bulk materials in a transistor configuration for the detection of gases such as hydrogen (H2), hydrogen sulfide (H2S), ammonia (NH3), nitrogen dioxide (NO2), water vapor, and methane. Other nanomaterials with semiconductive properties have shown promise for use as gas sensors.
The sensing mechanism of a semiconductive nanomaterial-based gas sensor depends upon charge transfer between the electron-donating/electron-withdrawing molecules of the gas and the semiconducting nanostructures. Such electron donation or withdrawal changes the conductivity of the nanomaterial. Nanomaterial-based sensors, therefore, using low-power microelectronics technology, convert chemical information into an electrical signal, leading to the formation of miniaturized sensor devices.
However, bare nanomaterial-based sensors do not exhibit high sensitivity toward certain gases due to their low absorption capacity. This less-than-ideal sensitivity, as well as relatively low selectivity, has limited the use of nanomaterial-based gas sensors in practical applications.
Current methods for using some nanomaterial-based gas sensors require highly reactive reagents and high temperatures to modify the nanomaterial structure and to make the materials suitable to act as gas sensors. In addition, functionalization of the sensors requires long fabrication time and complicated fabrication steps, which makes the process complex and costly. These functionalization techniques also have limited spatial resolution, which makes the creation of high density sensor arrays difficult.
Carbon nanotubes (CNTs) and one dimensional nanostructures such as nanowires have been demonstrated as appealing sensing materials for developing advanced chemical gas sensors. Based on the mechanism of charge transfer, gas adsorption (e.g., NO2, NH3, O2) can cause significant electrical transport property changes in the CNTs and nanowires. Compared to traditional thin film or thick film sensing materials. CNTs and nanowires offer several advantages, such as good sensitivity, room temperature operation, and fast response time due to their quasi one-dimensional structure, large surface area-to-volume ratio, small size, and unique electrical properties. However, the number of analytes that can be detected by pristine CNTs is limited to only a few types, such as CO and hydrogen.
Relative humidity (RH) is one major environmental factor that affects performance of most chemical gas sensors, including CNT sensors. For example, experimental measurements have demonstrated that some SWNTs and some multi-walled carbon nanotubes (MWNTs) exhibit increased resistance when interacted with atmospheric moisture (water vapor) in non-condensing conditions. One explanation for this increased resistance is that electron-donating water molecules deplete the hole charge carriers of p-type CNTs, thereby increasing the resistivity of the CNTs. Thus, minimizing or eliminating the RH effect is desirable for the reliable application of CNT gas sensors.